Add SCX_LAVD scheduler

[Yury-MonZon]

Kernel (configs/minconfig-mainline):

• CONFIG_SCHED_CLASS_EXT=y (sched_ext)
• CONFIG_DEBUG_INFO_BTF=y, CONFIG_DEBUG_INFO_REDUCED off (BTF is required by SCHED_CLASS_EXT and by the scx BPF programs)
• CONFIG_FTRACE_SYSCALLS=y (sys_enter_execve tracepoint used by scx_lavd)

Build environment:

• Add dwarves (pahole) to Dockerfile and README kernel deps; BTF generation needs pahole >= 1.22 or olddefconfig silently drops it
• build-kernel-mainline.sh: fail fast if pahole is missing/too old, instead of quietly shipping a BTF-less (sched_ext-less) kernel

Image (scx served from our apt repo):

• Enable scx.service via multi-user.target.wants overlay symlink

Benchmarks (RK3576, 8-core big.LITTLE, on battery, screen off, scx_lavd v1.1.1 --autopower vs stock EEVDF):

Latency — schbench -m4 -t8 -r30 (wakeup latency):
p50 3532 us -> 26 us (135.85x faster)
p90 16112 us -> 3988 us (4.04x faster)
p99 31776 us -> 7976 us (3.98x faster)
p99.9 43200 us -> 18272 us (2.36x faster)

Throughput: -23% RPS under full saturation (latency/throughput trade).

Power — average watts at fixed load (long-settle fuel-gauge sampling):
idle 3.32 -> 3.50 W (+5%, scx housekeeping prevents deepest idle)
2c @ 25% 3.70 -> 3.56 W (-4%)
4c @ 50% 4.87 -> 4.06 W (-17%, core compaction at partial load)
8c @ 100% 6.02 -> 5.89 W (~tie)

Net: large interactivity/latency win; meaningful power savings across the active partial-load range (peak ~17% at moderate load), small idle penalty.

NOTE: tests were run on a CPU without a heatsink. Sustained and high-load figures (esp. 8c@100%) may be influenced by thermal throttling and should be treated as indicative rather than absolute; the relative scheduler comparison holds since both modes were tested under identical conditions.

Downside: shows deprecation warnings on start, should be resolved in next version

[   13.989800] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   13.989811] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   14.949192] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   14.949210] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   14.953290] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   14.964833] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   14.966190] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   15.566527] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   15.635733] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.420296] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.421254] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.425207] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.426421] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.437729] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.438755] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.439759] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.440735] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.441694] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.488458] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.489390] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.501867] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.502901] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.504154] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.548835] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.550813] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.562714] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.563784] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.564993] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.883569] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.884404] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.895400] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.896430] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.897677] sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()
[   17.956087] NOTICE: Automounting of tracing to debugfs is deprecated and will be removed in 2030
[   17.961665] ops->cpu_acquire/release() are deprecated, use sched_switch TP instea